Virtual memory decouples the logical address space a process sees from the physical RAM installed in the machine. Each process gets its own contiguous virtual address space (e.g., 4GB on 32-bit systems), even if physical RAM is only 1GB. The OS maps virtual pages to physical frames on demand.
Why Virtual Memory
- Run larger programs: a 3GB executable can run on a 1GB machine
- Isolation: each process has its own address space — cannot see or corrupt other processes’ memory
- Efficiency: only the actively used parts of a program are in RAM
- Simplified linking: all programs link at the same virtual addresses
Demand Paging
Pages are loaded into RAM only when needed — not upfront. When a process accesses a virtual address whose page is not in RAM, a page fault occurs.
Page Fault Handling
- Hardware detects invalid page → traps to OS
- OS checks if the address is valid (vs segmentation fault)
- OS finds a free frame (or evicts a page)
- OS initiates disk I/O to read the page into the frame
- OS updates the page table entry
- OS restarts the faulting instruction
Page faults are expensive — disk I/O takes millions of CPU cycles. The goal of virtual memory management is to minimize page faults.
Thrashing
Thrashing occurs when a process spends more time paging (handling page faults) than executing. The disk is constantly busy, but no useful work gets done.
Cause: the sum of the working sets of all processes exceeds physical memory. Each process doesn’t have enough frames to hold its active pages, so it constantly faults.
Solution: reduce the degree of multiprogramming — swap out entire processes to free frames. This actually increases CPU utilization because the remaining processes stop thrashing.
Working Set Model
The working set is the set of pages a process is currently using (bounded by the locality of reference). If the OS keeps the sum of all working sets ≤ available frames, thrashing is avoided.
| Concept | Meaning |
|---|---|
| Temporal locality | Recently accessed pages will be accessed again soon |
| Spatial locality | Pages near recently accessed pages will be accessed |
| Working set | Pages in the current locality |
Q: Why do we need virtual memory?
A: (1) Run programs larger than physical RAM. (2) Process isolation — one process can’t corrupt another. (3) Efficient memory use — only active pages are in RAM. (4) Simplified programming — no need to manage physical memory layout.
Q: What happens during a page fault?
A: The MMU triggers a trap. The OS validates the address, finds a free frame (or evicts one), reads the page from disk into the frame, updates the page table, and restarts the instruction. A page fault takes millions of cycles — disk I/O is orders of magnitude slower than RAM.
Q: What is thrashing and how is it solved?
A: Thrashing is when a system spends most of its time paging rather than executing. Caused by too many processes competing for too few frames. Solved by reducing the degree of multiprogramming — swapping out processes until the remaining ones fit comfortably in memory.
Q: What is demand paging?
A: A virtual memory strategy where pages are loaded into memory only when accessed (faulted in). This minimizes startup time and wastes no RAM on code paths that are never executed.
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